The process of the present invention relates to a process for purifying an aqueous reaction and/or washing medium which contains cellulose ethers.
Cellulose ethers are generally produced by reacting cellulose, an alkali metal hydroxide and at least one etherifying agent in an aqueous medium which optionally contains one or more organic solvents. After separation of the produced crude cellulose ether the remaining aqueous reaction medium generally contains various impurities, such as hot water-insoluble cellulose ethers as well as hot water-soluble cellulose ethers and/or macromolecular cellulose ether by-products. The crude cellulose ether is usually washed with hot water and/or steam for purification purposes. After this washing step the washing medium also contains various impurities, such as hot water-insoluble cellulose ethers as well as hot water-soluble cellulose ethers and/or macromolecular cellulose ether by-products. The reaction and/or washing medium is usually purified in a biological water purification plant. Unfortunately, the cellulose ethers are poorly biodegradable and cause various problems in the biological water purification plant, such as foaming, poor settling of the biomass or even flotation and loss of the biomass. Accordingly, it would be desirable to pre-treat at least a portion of the reaction and/or washing medium originating from cellulose ether production before feeding the medium into a biological water purification plant.
U.S. Pat. No. 4,762,113 suggests purification of liquid media which comprise low-, medium- and high- boiling organic by-products and low-, medium- and high boiling, residual, non-reacted organic reaction compounds produced from the preparation of cellulose ethers, an alkali metal hydroxide and at least one etherifying agent in an aqueous medium. The purification is carried out in two steps. First the low-boiling organic by-products of the reaction and residual, non-reacted, low-boiling organic reaction components are separated by distillation. The aqueous distillation residue is treated by ultrafiltration. A polyether sulfone membrane is recommended for the ultrafiltration step. In the ultrafiltration step the aqueous distillation residue is separated into a permeate, having a reduced COD value, and a concentrate which substantially comprises undesirable salts and cellulose constituent. The U.S. patent teaches that the concentration of the concentrate should not be raised to a level which is too high since with increasing concentration the retained amount of compounds accounting for the COD value decreases and practically approaches zero at concentration ratios in the range from about 20 to 27, i.e. the COD values of the permeate approximately re-approach the COD value of the initial waste water. With the ultrafiltration method taught in the U.S. patent it is apparently not possible to achieve higher concentration ratios than about 20 to 27. The U.S. patent teaches that the permeate leaving the ultrafiltration process has a COD value which is reduced by about 20% and is directly fed into the waste-water purifying plant. The concentrate is spray dried and the solid residue of the concentrate can be further processed.
It is known that certain cellulose ethers, such as methyl cellulose ethers, ethyl cellulose ethers, methyl hydroxypropyl cellulose ethers and other known hot-water insoluble cellulose ethers are soluble in cold water. They increase the viscosity of an aqueous medium at low temperatures. An increase in viscosity causes a decrease in the efficiency of the ultrafiltration treatment disclosed in U.S. Pat. No. 4,672,113 due to a decreased flux. Furthermore, the relatively high molecular weight hot water-insoluble cellulose ethers tend to plugging or blocking up of the membrane used for ultrafiltration. An inefficient purification of aqueous media originating from cellulose ether production is very disadvantageous. The worldwide yearly production of cellulose ethers is over 100,000 tons per year. The production on such a large scale results in huge amounts of aqueous reaction and/or washing media which have to be purified. The suggested ultrafiltration method with a maximum concentration ratio of about 20 to 27 still leaves huge amounts of concentrate which have to be dried or disposed of in another way. The drying of such large amounts of concentrate is very expensive and energy-consuming.
In order to increase the efficiency of the ultrafiltration treatment disclosed in U.S. Pat. No. 4,672,113, European patent application EP-A-0,545,426 suggests purification of liquid reaction and/or washing media resulting from the production of hot water-insoluble cellulose ethers by ultrafiltration at a temperature above the flocculation temperature of the cellulose ether, preferably above 40.degree. C. It is recommended to subject the reaction and washing media first to a distillation in order to separate low-boiling organic components from the reaction and washing media first to a distillation in order to separate low-boiling organic components from the reaction and washing media. The aqueous distillation residue is treated by ultrafiltration, preferably by means of a polyether sulfane membrane. The concentrate of the reaction and/or washing media is purified by flocculation of the slowly degradable organic components.
Aqueous solutions of certain cellulose ethers, such as methyl cellulose ethers and hydroxypropyl methyl cellulose ethers, are known to gel upon heating. These gels are reversible in that they are formed upon heating yet will liquify upon cooling. Thermal gelation of these cellulose ether solutions has been known for a long time as "syneresis". U.S. Pat. No. 2,791,594 suggests to form a wet mixture comprising 35 to 2 wt.-%, preferably from 25 to 10 wt.-%, of fibrous cold-water-soluble cellulose ether and from 65 to 98 wt.-%, preferably from 75 to 10 wt.-%, of hot water at a temperature above the gel point of the cellulose ether. The mixture is then cooled below the gel point until the fibrous structure disappears, then the temperature is raised to a point above which syneresis occurs. After syneresis has occurred, water is removed by decanting, centrifuging or other mechanical methods before drying of the cellulose ether is started.
The process disclosed in German Offenlegungsschrift DE-A-41 34 662 (equivalent to WO 93/08217) makes use of the above-discussed syneresis. The German Offenlegungsschrift relates to a method of separating cellulose ethers and other cellulose derivatives from a washing medium which results from the purification of crude cellulose ether. In a first step flocculated cellulose derivative is removed from the washing medium which has been heated to a temperature above the gel point of the cellulose ether. Partial removal of the flocculated cellulose derivative is conducted in a settling tank. Non-settled gelled cellulose derivative is removed by centrifugal separation in a cyclone, preferably a hydrocyclone, or by flotation. In a second step, the remaining solution is cooled to a temperature below the gel point and is subjected to a membrane filtration. In the membrane filtration the volume ratio of concentrate to permeate is from 0.5 to 5, preferably about 1. Ultrafiltration through a poly(vinylidene flouride) (PVDF) membrane is recommended. The obtained concentrate is heated above the gel point, is further concentrated and is recycled to the first step.
Unfortunately, the separation of flocculated cellulose derivative from the washing medium in the manner suggested by German Offenlegungsschrift teaches is time-consuming and inefficient. The settling tank has to be heated in order to avoid that the flocculated cellulose derivative goes into solution at a decreased temperature. Furthermore, settling in the settling tank takes unduly long. Trials by the applicants have further revealed that the percentage of gelled cellulose ether that can be separated from the washing medium by means of a hydrocyclone varies significantly from one batch to the other. The percentage of various methyl cellulose ethers and hydroxypropyl methyl cellulose ethers that can be gelled and separated from the washing medium has been found to vary between from 37% to 97%. Further, the above-mentioned volume ratio of 0.5-5, preferably 1, corresponds to a concentration ratio (volume of membrane feed divided by volume of concentrate) of 1.2 to 3, preferably about 2. A membrane filtration with a such a low concentration ratio leaves huge amounts of concentrate which have to be disposed of or recycled to the production process. German Offenlegungsschrift DE-A-41 34 662 suggests recycling of the concentrate to the settling tank utilized in the first step. The concentrate contains water-soluble cellulose ethers as well as organic and inorganic impurities like salts which substantially affect the quality of the cellulose ether which is flocculated in the first step. Accordingly, the problem of handling the concentrate left after the membrane filtration is still not solved.
Accordingly, one object of the present invention is to find another method for purifying aqueous reaction and/or washing media containing cellulose ethers. A preferred object of the present invention is to find a purification process which includes a membrane filtration step that does not fail at a concentration ratio of about 20 to 27.